Generally, the piston rings that are provided in an internal combustion engine such as a diesel engine have an anti-seize protective coating provided on a sliding surface thereof that slides against the cylinder liner. Conventionally, multi-layer chrome plating has been commonly used for this protective coating, however, in recent years, due to changes in the sliding environment of the piston rings of internal combustion engines, instances in which seizures have occurred in the actual machinery even while the multi-layer chrome plating forming the protective coating is still extant are being seen more and more often. It is thought that this is due to the fact that, while multi-layer chrome plating makes it possible for a uniform coating to be formed, it is difficult for microscopic seizure portions in the coating to drop off, with the result that there is a tendency for a seizure to spread suddenly over a wide area.
In Patent document 1 (see below), a piston ring that is provided with a thermal spray coating (i.e., a cermet thermal spray coating) instead of multi-layer chrome plating as a protective coating is disclosed. This thermal spray coating has a hard phase formed from hard particles of tungsten carbide (WC) and chromium carbide (Cr3C2) and the like, and a metallic binder phase that is formed from molybdenum (Mo), cobalt (Co), nickel (Ni), and chrome (Cr). It is easier for microscopic portions of the coating to fall of in this thermal spray coating than in multi-layer chrome plating, so that a thermal spray coating has superior seizure resistance.